Light trickery makes bird feathers blue but not red

It’s a colourful puzzle. Blue jays and cardinals get their vivid hues of blue and red, respectively, in different ways – and efforts to understand why may help create coloured displays for devices such as e-readers.

Colours in nature normally come from pigments, which absorb most light wavelengths except those they reflect to give the colour we see. But blue pigment is rare. Instead, the blue sported by bluebirds comes from tiny air pockets inside the feathers, which scatter light to create blue. Red feathers, however, rely on pigment alone.

Vinothan Manoharan at Harvard University and his colleagues wondered if red colours were impossible to achieve without pigment. “We thought, maybe the birds know something we don’t,” he says. The team studied nanometre-scale plastic beads – inverse versions of the air pockets of blue feathers. Changing the size of beads alters how light scattered from nearby ones interferes, enhancing particular shades.

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Manoharan’s team used large beads to try to enhance red light, but purple appeared instead. The unexpected blue component appears for the same reason the sky is blue, Manoharan says&colon; individual particles preferentially scatter blue light.

This experiment shows why beetles, for instance, can’t make red hues unless they have the pigments to do so, because the way light bounces off beads is not dissimilar to how light would bounce off the microscopic structure of a beetle’s back.

But it cannot rule out a possible way to get red hues in feathers from light scattering, because feather colour comes from air pockets rather than from bead-like structures surrounded by air.

Richard Prum at Yale University in New Haven, Connecticut, has an explanation for why birds couldn’t use holes in their feathers to make reds even if they wanted to&colon; light can scatter multiple times within a feather, enhancing other colours that wash out the red.

Manoharan’s group is now seeing if hollow beads can form reds, and hopes to use such structural colours in reflective displays that aren’t backlit – think a Kindle, but in colour.